Many of the most effective drugs used in cancer chemotherapy are DNA- damaging agents. However, long-term survivors of treatment with at least some of these drugs have a substantially increased risk of developing second, unrelated malignancies, presumably as a result of the mutagenic effects of these drugs. The ultimate goal of the proposed studies is to understand the mechanisms by which certain antitumor drugs exert their mutagenic and carcinogenic effects, in order to facilitate the selection and development of more effective and less carcinogenic chemotherapeutic agents and protocols. For drugs which induce DNA double-strand breaks, including the radiomimetics bleomycin and neocarzinostatin and the topoisomerase II inhibitors m-AMSA and teniposide, the relationship between double-strand break repair and deletion mutagenesis will be investigated. Defined repair substrates which incorporate a unique site- specific double-strand break, with termini characteristic of particular drug-induced breaks, will be constructed. These substrates will be introduced into various mammalian and other eukaryotic cells, and repaired products will be recovered and analyzed in an attempt to develop models to explain how the double-strand breaks were repaired, how the blocked termini were processed, and whether any specific blocked termini were particularly prone to cause deletions during repair. In order to determine whether similar repair events occur in endogenous genes, mutations induced by these same drugs in the aprt gene in CHO cells will be sequenced. For the bifunctional alkylating agent melphalan, which has been strongly associated with second malignancies, sequences have been identified in CHO/aprt and in shuttle vector systems which are frequent sites of drug- induced base substitutions mutations. Both monofunctional and bifunctional adducts induced by the drug at these sequences will be identified in order to determine whether any adducts are specifically formed at these sites, and whether these adducts can be implicated in mutagenesis. Spectra of mutations induced by a monofunctional analogue of melphalan will be determined in CHO/aprt and in a shuttle vector system in order to determine the relative importance of monofunctional and bifunctional alkylation in mutagenesis. For all the drugs being studied, molecular computer graphics modeling will be employed in an attempt to explain in structural terms the types of DNA damage induced by each drug.